Imaging mass spectrometry (MS) is currently in its translational phase as a tool in medical histopathology. Many biological applications are being pursued due to its capability to provide comprehensive information on the distribution of multiple endogenous and exogenous molecules within animal tissues (van Hove E R A, Smith D F, & Heeren R M A (2010), J. Chromatogr. A 1217(25):3946-3954; Watrous J D, Alexandrov T, & Dorrestein P C (2011), Journal of Mass Spectrometry 46(2):209-222). Imaging MS has the capability of mapping drugs, metabolites, lipids, peptides and proteins in thin tissue sections with high specificity and without the need of fluorescent or radioactive labeling normally used in histochemical protocols (Schwamborn K & Caprioli R M (2010), Mol. Oncol. 4(6):529-538; and Chughtai K & Heeren R M A (2010), Chem. Rev. 110(5):3237-3277).
Within the imaging MS techniques (Alberici R M, et al. (2010), Analytical and Bioanalytical Chemistry 398(1):265-294), ambient ionization techniques such as desorption electrospray ionization mass spectrometry (DESI-MS) have been rapidly emerging and have the advantage of being performed at atmospheric pressure without the need for sample preparation (Ifa D R, Wu C P, Ouyang Z, & Cooks R G (2010), Analyst 135(4):669-681). Other imaging techniques such as matrix assisted laser desorption ionization (MALDI; Oppenheimer S R, Mi D M, Sanders M E, & Caprioli R M (2010), Journal of Proteome Research 9(5):2182-2190) and secondary ion mass spectrometry (SIMS; Fletcher J S & Vickerman J C (2010), Analytical and Bioanalytical Chemistry 396(1):85-104) are commonly performed under high-vacuum conditions and the former requires careful sample preparation through the application of a matrix. More recently, much effort has been put towards advancing ambient imaging mass spectrometry within the biomedical field, especially in cancer diagnostics (Dill A L, Eberlin L S, Ifa D R, & Cooks R G (2011), Chemical Communications 47(10):2741-2746). The prospect of improving the accuracy of histopathological cancer evaluation by adding chemical information to the morphological microscopic analysis, especially related to cancer diagnosis and grading represents an attainable and relevant medical application. Nonetheless, technical challenges remain and validation studies are still needed to successfully merge microscopic and mass spectrometric information into routine histopathology workflow.
As the ability of DESI-MS as a diagnostic tool is demonstrated in many studies, this capability must be validated through extensive chemical and microscopic examination of tissue sections and development of classification rules relating MS imaging molecular information to traditional pathology. The correlation between histology and DESI-MS has thus far been performed by comparing the ion images obtained to the diagnosis from pathological evaluation of a serial hematoxylin and eosin (H&E) stained section (Masterson T A, et al. (2010) Distinctive Glycerophospholipid Profiles of Human Seminoma and Adjacent Normal Tissues by Desorption Electrospray Ionization Imaging Mass Spectrometry. J. Am. Soc. Mass. Spectrom. in press). Even though this strategy is sufficient for optical image evaluation under routine microscopic pathology, workflow practicality and unambiguous correlation demand the use of the same tissue section for morphological and MS imaging evaluation.
The first strategy reported for conducting histopathology and imaging MS analysis on the same tissue section was the development of MALDI imaging compatible dyes, which provide limited histological details (Chaurand P, et al. (2004), Analytical Chemistry 76(4):1145-1155). Tissue section staining after MALDI imaging spectra acquisition and matrix removal is considered the most promising approach to pair MALDI imaging and histological staining, a strategy named post-acquisition staining (Crecelius A C, et al. (2005), Journal of the American Society for Mass Spectrometry 16(7):1093-1099). As an ambient imaging MS technique, DESI-MS frees the user from the need of a homogeneous matrix deposition on the sample.
A limitation preventing DESI-MS imaging compatibility with histochemistry is that the most common DESI solvent systems, methanol/water and acetonitrile/water 1:1 (v/v), completely destroy the native tissue morphology during analysis.